This invention relates to a method of cleaning and protecting a metal surface from rust and simultaneously applying to it a corrosion-resisting composition by subjecting the surface to an abrasive treatment with an abrasive the particles of which are coated with a binder and a corrosion-resisting agent.
A method of the type described above is known from the British Patent Specification No. 1,377,484, in which it is proposed to coat abrasive particles with a binder and a protective metal such as zinc, aluminium, cadmium or lead as a corrosion-resisting agent. Zinc in particular is used as the protective metal.
In the shipbuilding industry and in the manufacture of steel structures, especially where the steel is subjected to a rust-proofing treatment prior to being subjected to further operations, there are disadvantages to the use of zinc as a corrosion-resistant metal. For example, the fumes developed during autogenous or electric cutting and/or welding of steel protected with zinc are injurious to health. The welder, for instance, is then obliged to wear a helmet through which purified air is supplied, which form of protection is, however, a nuisance. Moreover, the workshops must be intensively ventilated. Besides, cutting steel provided with a zinc coat is slower than that of cutting untreated steel or of steel coated with a zinc-free primer.
Although the use in the above-proposed method of, for instance, zinc as a protective metal leads to effective protection against corrosion of the metal surface activated by the abrasive treatment, it has been found that during storage in the open air the metal surface will after some time show a white deposit as a result of the formation of zinc oxidation products such as zinc oxide, zinc hydroxide, and zinc carbonates (so-called "white rust"). When subsequently the metal surface is to be painted, first this white deposit must be removed entirely, which calls for an additional, labor intensive and accurate treating step. In practice it is found that the removal of the deposit is either incomplete or leads to damage of the thin protective metal layer. If the white deposit is not or is only partly removed, the adhesion of the paint system will deteriorate and blisters will be formed, particularly when the paint system is exposed to an aqueous medium such as water of condensation and aqueous solutions of chemicals. Another disadvantage of the known method of coating a metal surface with a zinc coating having a high zinc powder content is that the generally relatively low mechanical strength of the zinc layer will lead to the subsequently applied paint also being mechanically weak, so that it will be damaged easily, for instance, by falling tools, as a result of which there may be additional disengagement of the paint coating.